1. Field of the Invention
The present invention relates to a method, and apparatus for modifying a color television signal, to include additional signal information on a chrominance subcarrier.
2. Description Relative to the Prior Art
A fundamental property of conventional magnetic tape recording systems is that the amplitude of the signal obtained on playback is proportional to the rate of change of flux intercepted by a playback head. Such rate is in turn proportional to the speed of the tape relative to the head, and the frequency of the signal recorded on the tape. One result of this property is that the ratio of signal to noise deteriorates rapidly as the frequency falls, and such deterioration presents difficulties in video tape-recording, where the frequencies to be recorded typically cover the range of 20 Hz to 5 MHz, a ratio of 1:2.5 .times. 10.sup.5.
A known solution to this problem is to modulate the video signal on a carrier frequency so as to reduce the ratio between the upper and the lower frequency extremes to be handled. For example, if amplitude modulation of a 6 MHZ carrier were employed, the range of frequencies in the lower sideband would be approximately 1 MHz to 6 MHz, a ratio of 1:6. If the upper sideband alone is considered, the ratio is 1:1.83 (6 MHZ to 11 MHZ). There are of course difficulties in handling the resultant higher frequencies, and so there are limitations. Further, the simple harmonic relationship between luminance components, and between luminance and chrominance components is destroyed by the modulation process, and non-linear distortion becomes more serious.
It is usual to frequency modulate the carrier, though this requires a wider bandwidth than amplitude modulation, and spurious signals may be caused by "folding" of the higher order lower-sidebands around the zero-frequency axis, giving rise to patterning noise and moire' effects. Nonetheless, the majority of existing designs are of this type.